A variety of panel mountable fuse holders exist which are designed to insertingly accept and retain cylindrical cartridge fuses. Such holders are in general cylindrical in shape and have a panel mountable body made of insulating material. The body itself is usually provided with a threaded shoulder on the front end for mounting to a panel through a hole, a matching nut being thereafter slipped over the body and rotated into engagement along the threads. An axial cylindrical passage entering the body from the front of the fuse holder is configured to accept inserted fuses in a completely enclosed manner, the front of the holder being sealed shut by a fuse carrying rotary engaging knob assembly of some form, the knob typically containing a sleeve for receiving and holding the front terminal of the inserted fuse. A rear contact at the rear of the cylindrical passage communicates with an electrical connecting lug on the outside of the fuse assembly for lead attachment. Electrical connection with the forward end of the inserted fuse is typically achieved by a side terminal assembly positioned about and within a central section of the fuse holder body, the side terminal assembly being characterized generally by one or more conducting elements wholly or partially within the fuse holder body configured to engage the conducting sleeve in the knob assembly by rotary engagement as the knob assembly is rotated into a locking position. Additional compression means are typically provided for urging the fuse either into the conducting sleeve of the knob assembly or alternatively against the end terminal at the rear of the fuse passage in the body of the fuse holder. This is most typically effected by employing a spring associated either with the sleeve or with the end contact of the holder. Such compression means are normally considered essential to insure adequate contact with both ends of the inserted fuse so as to keep contact resistance at a minimum in order to avoid overheating the fuse itself, thereby effectively lowering its amperage rating.
Prior art fuse holders of this general type have a variety of disadvantages, mostly in cost, complexity, and overall size. In such fuse holders, end terminal assemblies are commonly axially inserted either from inside or outside the fuse holder body, carrying some form of lug at the outer end for lead attachment thereto. If inserted from the inside of the fuse holder body, a fundamental limit is placed on the size and shape of the end terminal if it is to pass through the exit passage at the end of the body. If inserted from outside the body, an additional element of the assembly is necessary to be disposed inside to provide adequate area for electrical contact, thereby adding to the cost of the assembly. It is an object of the invention to provide in the fuse holder an inexpensive one-piece end terminal contact of adequate interior dimension to provide adequate electrical contact to the inserted fuse.
It is desirable that an end terminal having a selected terminal lug configuration be readily insertable into a standard fuse holder body or, once assembled into the fuse holder body, be readily removable. If such a feature is provided, then an error in terminal lug specification on the manufacturer's part or an error in description on the customer's part may quickly be rectified, a feature frequently unavailable in more complex end terminal assemblies, which frequently involve irreversible riveting or staking operations in their manufacture. Such a feature would also be useful in the reconfiguration of existing installations by electricians, where a change of terminal lug form would accommodate, for example, an extra lead. Accordingly, it is an object of the invention to make the inserted end terminal readily removable and replaceable, preferably by simple means requiring no special tools.
As previously mentioned, contact is made to the front end of the inserted fuse by means of a conducting sleeve carried in an insulated knob assembly, the sleeve penetrating into the axial passage in the fuse holder body to contact by rotary engagement elements of the side terminal and make electrical contact thereto, whereby both the fuse terminals are placed safely behind the mounting panel so as to minimize shock hazards arising from accidental contact with the exterior elements of the fuse holder terminals. One side terminal design heretofore developed is in the form of a multi-piece assembly, involving a sleeve-like element inserted into the bore passage of the fuse holder body and held in place by mechanical engagement with an externally mounted ring positioned about the central region of the fuse holder body and penetrating therethrough so as to capture and contact the side terminal sleeve.
Side terminals configured for such engagement suffer generally from complexity, requiring a multiplicity of components in their assembly, as well as lack of reconfigurability, in that the side terminal is typically irreversibly inserted during fuse holder assembly by a crimping or staking operation. Thus, once assembled, a different terminal lug assembly cannot be attached unless additional system complexity is added e.g., separate terminal lugs of varying configuration and an associated attachment means to some form of universal side contact assembly All such system complexity adds to manufacturing costs.
A simple one-piece side terminal element for engaging the fuse holder sleeve would represent a substantial cost economy in fuse holder manufacturing, and a reversible engagement means allowing a side terminal means of given lug configuration to be readily replaced by another would represent a significant cost economy for the same reasons previously set forth with respect to the end terminal
One approach to this problem is represented by a fuse holder currently marketed by the Bussman Company wherein a single piece side terminal ring is assembled to the fuse holder body by sliding it forward along the fuse holder body to engage the fuse holder body by snap-in engagement. Longitudinally extending unitary ring projections reach forward to snap into engagement with paraxial passages in an enlarged thread boss, whereby the contacting side terminal is held in place. Bladelike outwardly extending projections on the knob assembly sleeve engage slots in the side terminal ring through passages in the fuse holder body by rotational engagement, thereby completing the circuit from the side terminal ring to the outer end of an inserted fuse.
Such a terminal arrangement has the advantage that it is of the snap-in type, and may be reconfigured at will; however, the fact that the forward-reaching ring securing projections are in axial alignment with the fuse holder body requires that an oversized thread boss be provided to accommodate these securing elements, with the result that the overall diameter of the fuse holder is increased to an unnecessary degree, thereby reducing the density of fuse holder arrays that can be disposed along a given interval on the mounting panel.
Accordingly, a further object of the invention is to provide a one-piece side terminal contact using a snap-in insertion engagement without substantially increasing the overall diameter of the fuse holder, and which can be removed after assembly for reconfiguration as well.
A further problem encountered is that the cartridge electrical fuses available on the market are frequently found to have their end caps tilted substantially off-axis with respect to the central axis of the fuse body. To minimize contact resistance, some means must be provided to assure adequate electrical contact to such tilted end caps, without at the same time inordinately stressing them and running a substantial risk of fuse breakage. One commonly used means whereby this is accomplished involves self-aligning end cups or pistons used in conjunction with a compression spring. Such arrangements are frequently employed to improve contact bewtween the end terminal of the fuse holder with the inserted fuse. Such assemblies are typically expensive to fabricate, requiring a multiplicity of parts, and are also typically non-reconfigurable. Accordingly, it is an object of the invention to provide a simplified end terminal with improved contacting properties to fuses with tilted end caps while retaining the property of reconfigurability.
A similar contacting problem arises with respect to the knob assembly sleeve. Conventional solutions involve the use of one or more extra pieces in the form of a finger-contact of one form or another on the side of an inserted cup, or a similar contact formed by slotting the side of the sleeve to provide one or more such fingers. Such systems suffer either from complexity and concomitant manufacturing expense, or alternatively from poor accommodation to tilted fuse terminals. In general, those contacting systems which accommodate tilted terminals without unduly stressing them are complex. Accordingly, it is an object of the invention to provide a simple contacting system for the knob assembly sleeve which does not unduly stress an inserted fuse with tilted end terminals.
Another problem frequently encountered is the vulnerability of panel-mounting cartridge fuse holders to momentary power interruption if the knob assembly should accidentally be pushed inwards when in the locked condition. In many systems, particularly those involving volatile memory storage elements, such momentary power interruption can be catastrophic. Prior art fuse holder structures which avoid this problem typically do so by use of complex interconnection systems or by relying solely on an edge contact between the fuse holder members. It is a further object of the invention to provide an inexpensive secure side terminal contact that is essentially invulnerable to loss of contact under axial impact.